The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and particularly to the manufacture of cutter assemblies for use in such drill bits.
Rotary drill bits of the kind to which the present invention is applicable comprise a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of cutting elements. The cutting elements are so called "preform" cutters comprising a tablet, often circular or part-circular, made out of a thin hard facing layer, usually polycrystalline diamond, bonded to a less hard backing layer, usually cemented tungsten carbide.
Usually the bit body is machined from solid metal, usually steel, or is moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
In a steel-bodied bit, and also in some matrix-bodied bits, each cutting element is normally mounted on a carrier, such as a stud or post, and the stud or post is then secured within a socket in the bit body. Usually the rear surface of the backing layer is bonded to a surface of the carrier by brazing, although it has also been proposed that other bonding methods may be used, and in particular diffusion bonding.
In conventional two-layer cutting elements the rear surface of the backing layer is substantially flat and is bonded to a corresponding flat surface on the carrier. However, the less hard backing layer normally has a greater coefficient of thermal expansion than the hard facing layer of the cutting element and consequently when the element is heated to the temperature necessary to effect brazing or diffusion bonding there is a differential expansion which causes the cutting element to become dished, with the result that the rear surface of the backing layer is convexly deformed. Very high pressure requires to be imparted to the cutting element to flatten it against the flat surface of the carrier and this is difficult to achieve in practice. Consequently, the convex deformity of the rear surface of the backing layer has the result that the spacing between the rear surface and the surface of the carrier is not of constant thickness but is greater nearer the periphery of the cutting element and this may adversely affect the strength of the final bond.
The present invention sets out to provide methods for overcoming this problem.